1. Field of the Invention
The present invention relates to a plasma apparatus which generates plasma utilizing microwave discharge and performs laser excitation and can be utilized in plasma processing.
2. Description of the Prior Art
In the prior art, there are various plasma apparatuses utilizing microwave discharge, such as a laser apparatus, a light source apparatus, a plasma processing apparatus, an ion source or the like.
FIG. 1 is a sectional view of a gas laser apparatus in the prior art disclosed, for example, in Journal of Applied Physics, vol. 49, No. 7, July 1978, p. 3753, and FIG. 2 is a sectional view taken in line 2--2 in FIG. 1. In the figures, numeral 300 designates a waveguide for transmitting microwave, numeral 301 designates a waveguide taper provided on a portion of the waveguide 300, numeral 302 designates a laser discharge tube of Pyrex glass installed in a space of the waveguide taper, numeral 303 designates a laser gas inlet port provided on an end of the laser discharge tube 302, numeral 304 designates a laser gas outlet port provided also on an end of the laser discharge tube 302, numeral 305 designates a cooling gas feed tube installed to surround the laser discharge tube 302, numeral 306 designates a cooling gas inlet port provided on an end of the cooling gas feed tube 305, numeral 307 designates a cooling gas outlet port provided also on an end of the cooling gas feed tube 305, numeral 308 designates Brewster windows installed on both ends of the laser discharge tube 302, numeral 309 designates a cathode for DC discharge, and numeral 310 designates an anode also for DC discharge.
In the gas laser apparatus of the prior art as above described, a laser gas such as CO.sub.2 laser gas is introduced from the laser gas inlet port 303 into the laser discharge tube 302, and on the other hand microwave of TE1O mode is excited in the waveguide 300. Since the waveguide 300 has the waveguide taper 301 on the inside and the inner diameter of the waveguide 300 becomes minimum at the installation position of the laser discharge tube 302, the electric field intensity of the microwave becomes maximum in this position. The strong microwave field causes discharge breakdown of the laser gas within the laser discharge tube 302 and generates plasma and excites the laser medium. Then N.sub.2 gas of low temperature for example flows in the cooling gas feed tube 305 at high speed and the laser discharge tube 302 is cooled from outside and the discharge condition such as pressure of the laser gas is suitably selected whereby the laser oscillation condition is obtained, and further mirrors for laser oscillation (not shown) are installed on the outside of the Brewster windows 308 whereby the laser oscillation is performed.
In the gas laser apparatus of the prior art as above described, since the closed laser discharge tube 302 is used, if plasma with conductive property is generated, microwave mode of coaxial mode using plasma in the laser discharge tube 302 as the inner conductor becomes predominant and the microwave field in the plasma becomes electric field which component parallel to the tube wall of the discharge tube 302 is the main component, whereby microwave mode entering the plasma substantially perpendicular becomes to the tube wall of the laser discharge tube 302, i.e., the plasma boundary. In such discharge produced by the microwave being incident perpendicularly to the plasma boundary, the microwave field is decreased from the discharge tube wall towards inside, but since the discharge plasma has constant-voltage characteristics, the current density varies significantly depending on slight variation of the electric field. As a result, the plasma is generated and centered upon position near the discharge tube wall and becomes significantly uneven. This state is shown in a sectional view of FIG. 3. In FIG. 3, numeral 311 designates electric line of force of the microwave electric field, and numeral 312 designates plasma. In a gas laser apparatus utilizing microwave discharge in the prior art, since uneven plasma as shown in FIG. 3 is generated, it is difficult to make discharge as a whole suitable for the laser excitation. Moreover, the laser resonator mode and the plasma are not overlapped with each other and the laser output or efficiency becomes low.
In fact, in the apparatus of the prior art shown in FIG. 2, when microwave is 2.45 GHz and pulse microwave of 132 Hz is used and the apparatus is operated at pulse width 1 .mu.s and peak power 2.6 kw, mean output of only 15 mw is obtained. This is because the apparatus can be operated only at pulse width 1 .mu.s, 132 Hz, i.e., at very low pulse duty being about 1/10,000 due to unevenness of discharge as above described. Moreover, since the apparatus is operated at such slow repetition as 132 Hz, the plasma becomes uneven in aspect of time and therefore only laser oscillation by pulse can be performed. Such problems are produced not only in the gas laser apparatus, but various problems are produced also in other plasma apparatuses due to unevenness of discharge.
Also in the prior art, there is an optical waveguide type gas laser apparatus where laser gas is filled in a space acting as a light waveguide path to guide generated laser rays, and plasma is generated in the laser gas by discharge so as to perform laser excitation.
FIG. 4 is a sectional view of a optical waveguide type gas laser apparatus in the prior art disclosed, for example, in Japanese patent application laid-open No. 54-103692, and FIG. 5 is a sectional view taken line 5--5 in FIG. 4. In the figures, numerals 321, 322 designate a pair of long dielectrics opposite to each other, numerals 323, 324 designate a pair of long electrodes opposite to each other between these dielectrics 321, 322 with a prescribed spacing, numeral 325 designates a discharge space where all sides are defined by these dielectrics 321, 322 and electrodes 323, 324 and laser gas is filled therein so as to perform laser excitation by discharge, numeral 326 designates a block of material with high thermal conductivity, on which these dielectrics 321, 322 and electrodes 323, 324 are installed, numeral 327 designates a total reflection mirror arranged on one end of the discharge space 325, numeral 328 designates a partial transmission mirror arranged on other end of the discharge space 325 in opposition to the total reflection mirror 327, and numeral 329 designates a high-frequency voltage source for applying high-frequency electric field to the electrodes 323, 324. In this case, the discharge space 325 has dimensions suitable to guide the generated laser rays, and acts also as a light waveguide path.
Next, operation will be described. Laser gas is introduced into the discharge space 325, and high-frequency voltage is supplied between the electrodes 323 and 324 from the high-frequency voltage source 329. Thereby the strong high-frequency electric field is applied to the laser gas in the discharge space 325, and the discharge breakdown of the laser gas is produced by the high-frequency electric field and plasma is generated and the laser excitation is performed. The generated laser rays pass through the discharge space 325 as light waveguide path and are reflected between the total reflection mirror 327 and the partial transmission mirror 328 arranged on both ends of the discharge space 325, and a part of the laser rays is taken to the outside by the partial transmission mirror 328.
Since the optical waveguide type gas laser apparatus in the prior art is constituted as above described, it is difficult to make the frequency of the high-frequency voltage applied between the electrodes 323 and 324 high enough. Moreover, as the frequency of the high-frequency voltage is increased, the discharge in the discharge space 325 is concentrated toward surface portion of the dielectrics 321, 322 arranged on both sides of the discharge space 325, thereby uniform plasma cannot be obtained and it is difficult to make the discharge space 325 as a whole suitable for the laser excitation.
In any of the gas laser apparatuses in the prior art as above described, since the laser discharge tube as a whole is made of Pyrex glass, heat generated within the laser discharge tube cannot be radiated efficiently and the temperature rise of the laser gas such as CO.sub.2 gas causes decrease of the laser output.